I didn’t mean Molly hitting him. I meant Molly standing up to him. I love how fierce and hurt and angry she is there. It’s so rich and beautifully acted.
Assault, though (or battery maybe, not sure how that works in the UK), is dependent on consent. Somebody so much as making physical contact without your consent is assault. Doesn’t matter if it’s painful.
Of course, how you figure out whether it’s consensual could probably fill constellations. Just think of the nature of your physical contact with friends and loved ones. Patting, grabbing, hugging, leaning, slap-fighting… Sometimes you’re mad at first, but then decide it’s okay when you discover they had a good reason for doing it. Sometimes you wouldn’t normally be okay with it but you know you were kind of asking for it that time. Sometimes it’s okay from one person but not from another. When it comes to sex, we say yes means yes and no means no, but we can’t live every aspect of our lives that way. Far too much of human interaction is built on context and unspoken cues.
So basically it’s assault if Sherlock decides it’s assault.
Which is entirely unrelated to the moral okayness or lack thereof of physical violence as a method of communicating your anger. Sherlock’s anything but a fragile snowflake. If he figures Molly’s earned the privilege of smacking him in the face a couple of times, that’s his call. But expressing yourself via a fist in the face rather than, you know, talking about your hurt and anger (John, I’m looking at you here)—it may not be assault, but it’s still not healthy.